Semi-active suspension system with energy saving actuator

ABSTRACT

An actuator for a vehicle suspension system comprises a first member connectable to one of a sprung mass and an unsprung mass of the vehicle. The first member has an opening with an inside wall of predetermined dimension. A second member is connectable to the other of the sprung mass and the unsprung mass of the vehicle. The second member has an outside wall of a predetermined dimension. The dimension of the outside wall of the second member is adapted so as to be telescopically received within the opening of the first member thereby defining a fluid chamber. A source of pressurized fluid is connectable to the fluid chamber. A fluid seal is located near an end of the first member form a fluid seal between the first member and the second member. Bearings are operatively mounted between the inside wall of the first member and the outside wall of the second member for maintaining the inside wall of the first member spaced form the outside wall of the second member thereby defining a fluid manifold between the inside wall of the first member, the outside wall of the second member, the bearings, and the fluid seal. The bearings permits fluid pressure in the fluid chamber to communicate with the fluid manifold. The fluid manifold is connectable to reservoir. The continuous lubrication between sliding members and the low pressure seal provide an actuator with reduced friction which, in turn, reduces the system energy required to operate the suspension system.

TECHNICAL FIELD

The present invention is directed to an apparatus for controllingrelative motion between an unsprung mass and a sprung mass of a vehicleand is particularly directed to a semi-active suspension system havingan energy saving actuator.

BACKGROUND ART

Vehicle suspension systems are well known in the art. Such suspensionsystems have as their goal the control of the relative motion betweenthe sprung (chassis) mass and unsprung (associated tire) mass of thevehicle. Suspension systems are classified as either passive,semiactive, or active.

Passive suspension systems dissipate energy produced when a vehicle isdriven over an irregular road surface. Such systems provide goodvibration isolation. A linear response of a passive suspension systemcan be altered by (i) adding an advantageous nonlinear attribute, suchas direction dependant damping, and (ii) minimizing an objectionableeffect, such as stiction. Passive systems, however, react only toapplied forces from below through the road surface and from abovethrough inertia of the sprung mass or vehicle body. Ideally, asuspension system should appear "soft" in reacting to road noise inputsand stiff when reacting to inertia inputs. Since a passive system cannotdistinguish between the two, an engineering compromise is made.

An active suspension system uses power from the vehicle engine toactively move the vehicle wheels over an irregular road surface. Ratherthan a shock absorber, as is found in passive suspension system, anactive suspension system uses a hydraulic servo-actuator, i.e., ahydraulic motor, to move the vehicle wheel. A plurality of sensors arelocated at various vehicle locations. A controller, e.g., amicrocomputer, monitors the sensor outputs and controls operation of thehydraulic actuator through an electrically controlled servo valve.Through a control algorithm, the controller controls reaction to roadnoise and inertia inputs and controls relative motion of the sprung andunsprung masses.

In an active suspension system, the servo valve and controller functionas an energy control device. The servo valve connects the energy source,i.e., a pump, to the energy consumer, i.e., an actuator. The differencebetween power in and power out is converted to heat energy by the servovalve.

In a fully active suspension system, the actuator is operated so as tomove the wheel up and down relative to the vehicle body as necessary toprovide a desired "ride feel" and "handling characteristic" of thevehicle. The hydraulic pump provides energy in terms of fluid flow atsystem pressure. The servo valve removes energy at a rate to providefluid flow and pressure so as to move the wheel at a velocity needed toachieve the desired ride feel and handling characteristics. Control offluid flow controls actuator velocity. Control of fluid pressurecontrols actuator force.

In a typical fully active suspension system, each corner i.e., eachwheel, of the vehicle has an associated actuator and servo valve. Thepower consumption of each corner is the product of the fluid flow to theactuator times the supply pressure. Road noise occurs at highfrequencies. Large strut velocities are often required to prevent largeinputs from effecting commensurate motion of the vehicle chassis. Suchlarge velocities requires a large amount of energy. Since the hydraulicpump is driven by the vehicle engine, a large amount of energy consumedby the active suspension system means that a large amount of energy isbeing drained from the vehicle engine.

It is therefore desirable to develop a suspension system that providesbetter ride and handling control than a passive system but does notconsume the energy required by a fully active suspension system.

SUMMARY OF THE INVENTION

Inputs to a vehicle suspension system can be characterized as loadaiding and load opposing. Inputs are load aiding if the differentialpressure in the actuator is consistent with the drive signal or velocitydemand so that, if an orifice connected the two fluid chambers of theactuator together, the desired actuator motion would occur, i.e.,actuator motion would occur passively. For example, if an external forceon the actuator is acting in compression, and the drive signal is"telling" the actuator to compress, the actuator is said to be in a loadaiding situation.

Inertia loading is always load opposing and road inputs have both loadaiding and load opposing modes. Therefore, there are significantportions of the suspension system's duty cycle where a passive systemwould suffice. Furthermore, there are road input modes that are loadaiding and in which a fully active suspension system operates veryinefficiently.

The present invention provides an improved suspension system that issemi-active. The suspension system, in accordance with the presentinvention, provides active control of the suspension, i.e., connectionwith the pump, for low frequency input control and passive control,i.e., no connection with the pump, for high frequency input control.This arrangement provides energy saving in the operation of thesuspension system.

The present invention further provides an energy saving semi-activesuspension system with a low friction actuator design. A quiescent pilotfluid flow through a first stage of a servo valve is continuouslydrained to reservoir between adjacent walls of telescopic members thatmake the actuator. This arrangement keeps the actuator continuouslylubricated. The actuator includes only a low pressure fluid sealoperatively located between the telescopic members thereby furtherreducing sliding friction.

In accordance with one aspect of the present invention, an actuator fora vehicle suspension system comprises a first member connectable to oneof a sprung mass and an unsprung mass of the vehicle. The first memberhas an opening with an inside wall of predetermined dimension. A secondmember is connectable to the other of the sprung mass and the unsprungmass of the vehicle. The second member has an outside wall of apredetermined dimension. The dimension of the outside wall of the secondmember is adapted so as to be telescopically received within the openingof the first member thereby defining a fluid chamber. Means are providedfor connecting a source of pressurized fluid to the fluid chamber. Fluidseal means located near an end of the first member form a fluid sealbetween the first member and the second member. Bearing means areoperatively mounted between the inside wall of the first member and theoutside wall of the second member for maintaining the inside wall of thefirst member spaced from the outside wall of the second member therebydefining a fluid manifold between the inside wall of the first member,the outside wall of the second member, the bearing means, and the fluidseal means. The bearing means permits fluid pressure in the fluidchamber to communicate with the fluid manifold. Means are provided forconnecting the fluid manifold to reservoir.

In accordance with another aspect of the present invention, a vehiclesuspension system comprises an actuator connectable between chassis anda wheel, the actuator having a variable volume fluid chamber.Displacement between the chassis and wheel is controlled by pressure inthe actuator chamber. The suspension system further comprises a pump, anaccumulator, and means to pressurize the accumulator from the pump to apredetermined pressure. Sensing means are provided for sensing fluidpressure in the actuator chamber and in the accumulator, fluid pressurein the chamber is indicative of force inputs to the vehicle. Anelectrically controlled valve connects a selected one of the pump andthe accumulator to the actuator chamber. Control means are provided forcontrolling the electrically controlled valve in response to forceinputs and a pressure differential between the accumulator and theactuator chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention will become apparent to thoseskilled in the art to which the present invention relates from readingthe following specification with reference to the accompanying drawings,in which:

FIG. 1 is a schematic of an vehicle suspension system made in accordancewith the present invention;

FIG. 2 is a sectional side view of a preferred actuator arrangement madein accordance with the present invention;

FIG. 3 is an enlarged side view of a portion of the actuator shown inFIG. 2;

FIG. 4 is a schematic diagram of the servo valves shown in FIG. 1; and

FIG. 5 is a flow diagram of a control process for the vehicle suspensionsystem shown in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, a suspension system 10 for a vehicle includes fourcorners corresponding to the four wheel corners of the vehicle. Throughout this specification, the structure and operation of only one corneris described, it being understood that the structure and operation ofthe other three corners is similar. In the corner shown, a singleacting, linear actuator 12 operatively connects the sprung mass 14, i.e,the vehicle chassis, to the unsprung mass 16, i.e., the wheel hub andtire.

Referring to FIGS. 1-3, the actuator 12 includes a first member 18 inthe form of a cylindrical housing. The cylindrical housing 18 isconnected to the chassis 14. A second member 20 is telescopicallyreceived in the cylindrical housing 18. The second member 20 extendsthrough one end 26 of the cylindrical housing 18 and is connected to thewheel hub 16. The second member 20 and the cylindrical housing 18 form avariable volume fluid chamber 28.

The second member 20 is supported in the cylindrical housing 18 byspaced apart bearings 30, 32. Specifically, bearing 30 is received in arecessed opening 34 of the second member 20. Bearing 32 is received in arecessed opening 36 on an inside wall surface 38 of the cylindricalhousing 18. A low pressure seal 40 is located near the end 26 of thecylindrical housing 18.

The diameter of the outer surface of the second member 20 is less thanthe inner diameter of the inner wall surface 38 of the cylindricalhousing 18. The bearings 30, 32 keep the second member 20 uniformlyspaced from the inner wall surface 38 of the cylindrical housing 18 soas to form a cylindrical manifold chamber 42. The seal 40 is operativebetween the outer wall surface of member 20 and the inner wall surface38 of the cylindrical housing 18.

The first member 18 further includes a central cylindrical member 50secured at its end 52 to the upper wall surface 54 of the cylindricalhousing 18. The second end 56 includes an outwardly directed flangeportion 58 having a first stop surface 60. The second member 20 includesan inwardly directed flange portion 62 at its end 64. The flange 62includes a second stop surface 66. The flanges 58, 62 define the maximumextension of the second member 20 relative to the first member 18 asoccurs when the stop surfaces 60, 66 engage.

An end 68 of the flange 62 has a diameter greater than the diameter ofthe outer surface 70 of the inner cylindrical member 50 thereby forminga passage 72 therebetween. An end 74 of the flange 58 has a diameterless than the diameter of the inner surface 76 of the second memberthereby forming a passage 78 therebetween.

As mentioned, the actuator 12 is single acting. Fluid pressure in thechamber 28 acts on surfaces 80, 82 of the second member 20. Thepressurized fluid in the upper portion of the chamber 28 acts on thesurface 82 through passages 72, 78. If the fluid pressure in chamber 28times the surface area of the second member 20 acted upon by such fluidpressure in chamber 28 is greater than the load at the corner associatedwith such actuator, the second member 20 moves relative to the firstmember downward as viewed in FIG. 1. If the fluid pressure in chamber 28times the surface area of the second member 20 acted upon by such fluidpressure in chamber 28 is less than the load at the corner associatedwith such actuator, the second member 20 moves relative to the firstmember upward as viewed in FIG. 1.

A displacement sensor 90 is operatively connected between the firstmember 18 and the second member 20. It will be appreciated that any ofseveral known displacement sensors can be used to provide an electricalsignal having a characteristic indicative of the relative spacingbetween the first member 18 and the second member 20. The distancebetween the first member 18 and the second member 20 is, in turn,indicative of the relative distance between the chassis 14 and the wheelhub 16. In accordance with a preferred embodiment of the presentinvention, a linear voltage differential transformer ("LVDT") 90 has anelectric coil 92 secured to the cylindrical housing 18. The LVDT sensorfurther includes a ferromagnetic tube 94 secured to the second member20. The coil 92 is telescopically received in the tube 94. Theelectromagnetic field coupling between the coil and the ferromagnetictube varies as a function of how much of the coil is within the tube.The LVDT sensor provides an electric signal having a value functionallyrelated to the electromagnetic field coupling which is, in turn,indicative of the relative spacing between the first member 18 and thesecond member 20.

The chamber 28 is in fluid communication with an air over fluidaccumulator 100 through a first valve assembly 101 including anelectrically controlled valve 102. The chamber 28 is in fluidcommunication with a source of pressurized fluid 104 through a secondvalve assembly 105 including an electrically controlled valve 106. Thevalve assemblies 101 and 105 are preferably located in a single housing107 and secured to the side of the actuator 12.

The source of pressurized fluid 104 is preferably a variabledisplacement pump having a load sense input port 108. The output 110 ofthe pump 104 is fed back to the load sense input 108 via a pressuresensing line 112. The input of the pump 104 is in fluid communicationwith a reservoir 120.

It will be appreciated that, rather than a pressure controlled variabledisplacement pump, an electrically driven pump could be used wherein thepressure sensor 112 would be a circuit that would monitor the fluidpressure of the output 110 and would control the pump so as to maintaina constant predetermined output pressure. Also, a pump with anelectrically controlled pressure regulator operatively located betweenthe pump and the valve 106 could be used. The preferred embodiment ofthe invention includes a fluid pressure source of constant value at theinput of the valve 106.

The cylindrical manifold 42 of the actuator 12 is in fluid communicationwith the reservoir 120 through a conduit 122. The bearings 30, 32 aredesigned to permit fluid leakage therepass so that pressurized fluidpasses from the chamber 28 into the manifold 42 and out to the reservoir120. The pressure in the fluid manifold 42 will be only slightly greaterthan the reservoir pressure. Therefore, the seal 40 is preferably a lowpressure fluid seal. By having a fluid lubrication between the firstmember 18 and the second member 20 and by having a low pressure seal 40therebetween, the actuator 12 has a reduced friction as compared toknown actuators of other designs.

A position control device 130 is operatively coupled to the valve 102 soas to control the amount of fluid communication between the accumulator100 and the chamber 28 of the actuator 12 in response to an electricalcontrol signal. A position control device 132 is operatively coupled tothe valve 106 so as to control the amount of fluid communication betweenthe pump 104 and the chamber 28 of the actuator 12 in response to anelectrical control signal. A pressure sensor 134 is operativelyconnected to the accumulator 100 and provides an electrical signalhaving a value functionally related to the fluid pressure P_(A) in theaccumulator. A pressure sensor 136 is operatively connected to thechamber 28 of the actuator 12 and provides an electrical signal having avalue functionally related to the fluid pressure P_(i) in the actuator12.

A controller 140, such as a microcomputer, is operatively connected tothe pressure sensors 134, 136, the position controllers 130, 132 of theelectrically controlled valves 102, 106 respectively, and to thedisplacement sensor 90 for each of the four vehicle corners. Thecontroller 140 is further connected to a plurality of center-of-gravity("CG") sensors 142. These CG sensors 142 are typically located at ornear the center of the vehicle and detect low frequency motions of thevehicle. Such CG sensors 142 include yaw sensors, lateralaccelerometers, longitudinal accelerometers, etc. The controller 140monitors data output from all the vehicle sensors to which it isconnected and controls pressure in each accumulator 100 of each cornerand in the chamber 28 of each corner so as to control the vehiclesuspension in response to sensed high frequency road inputs and lowfrequency inertia inputs.

Referring to FIG. 4, the valve assembly 101, in accordance with apreferred embodiment of the invention, is an electrically controlled twostage servo valve and includes a second stage spool valve 102 and afirst stage flapper valve arrangement 150 that forms part of theposition control device 130. The position control device includes anelectric torque motor 152 that is operatively connected to the flappervalve 150. The torque motor 152 includes a magnetic assembly 154including magnetic coils 156, pole pieces 158, 160, and an armature 162.A flapper arm 164 is secured to the armature 162 and extendsperpendicular therefrom. The magnetic assembly 154 is secured to ahousing 170 of the valve 102.

The flapper 164 has a mounting location 174 about which a flexure tube176 pivotally secures the flapper 164 to the housing 170 through a pressfit. The flexure tube 176 functions to position the armature 162 betweenthe pole pieces 158, 160 and to provide a fluid seal between themagnetic assembly 154 and the flapper valve 150 and the spool valve 102.

The valve 102 includes a spool 180 slidably mounted in a chamber 182.Each side of the spool 180 has an associated pilot chamber 184, 186,respectively. The pilot chamber 184 is in fluid communication with thesupply pressure P_(s) through a fixed orifice 190. The pilot chamber 186is in fluid communication with the supply pressure P_(s) through a fixedorifice 192.

The pilot chamber 184 is in fluid communication with an associated pilotorifice 196. The pilot chamber 186 is in fluid communication with anassociated pilot orifice 198. The flapper 164 is positioned at alocation equal spaced from the pilot orifices 196, 198 when the armature162 is centrally located between the poles 158, 160. In such acondition, fluid flows through both pilot orifices 196 and 197 equallyso as to provide an equal pressure on both sides of the spool 180. Thespool 180 under such a condition assumes a centered position as shown inFIG. 4. Flow of fluid through the orifices 196, 198 is referred to asfirst stage flow.

The spool 180 includes spaced apart land sections 202, 204, spaced by aportion 206. When the spool is in its centered position as shown in FIG.4 the land 204 permits a relatively small fluid communication betweenchamber 182 and a passage 212, and passage 210 is in fluid communicationwith the area of chamber 182 between the lands 202, 204. A feed backwire 220 operatively connects the end of the flapper 164 with the land202. The passage 212 is in fluid communication with the fluid chamber 28of the actuator 12. The passage 210 is in fluid communication with theaccumulator 100.

The spool 180 when in the centered position as shown in FIG. 4, permitsa relatively small amount of fluid communication between the accumulator100 and the fluid chamber 28 of the actuator 12. When the flapper 164 isin the centered position, pilot fluid from the pump 104 communicatesthrough orifices 190, 192, through nozzles 196, 198, respectively,through passage 210 to the accumulator 100. This pilot fluid flow isreferred to as a quiescent fluid flow of the first stage valve. Also,because of the design of the actuator 12, there is a small leakage pathto the reservoir 120 through chamber 182, passage 212, chamber 28, andmanifold 42.

When the controller 140 applies an electric current to the coils 156 inone direction, the armature 162 pivots about the mounting location 174due to the magnetic attraction of the pole pieces 158, 160 on thearmature 162. This pivoting of the armature 162 causes the flapper 164to move toward one of the pilot nozzles 196, 198 depending of thedirection of pivot, i.e., the direction of the current through coils156. Since the valve 102 is only controlling the fluid communicationbetween the accumulator 100 and the chamber 28, the controller pivotsthe armature in a clockwise direction as view in FIG. 4. As the flapper164 moves closer toward the nozzle 196, the fluid pressure in chamber184 increases and the fluid pressure in chamber 186 decreases. Thispressure imbalance causes the spool 180 to move to the right, as viewedin FIG. 4, thereby permitting a greater amount of fluid communicationbetween the accumulator 100 and the chamber 28 of the actuator 12. Theamount of fluid communication is a function of the pressure imbalanceacross the spool 180 which is, in turn, a function of the amount ofcurrent through the coils 156. The feedback wire 220 tends to pull theflapper 164 away from the nozzle as the spool 180 moves in a manner wellknown in the art of two stage servo valves.

The valve assembly 105, in accordance with a preferred embodiment of theinvention, is an electrically controlled two stage servo valve andincludes a second stage spool valve 106 and a first stage flapper valvearrangement 250 that forms part of the position control device 132. Theposition control device includes an electric torque motor 252 that isoperatively connected to the flapper valve 250. The torque motor 252includes a magnetic assembly 254 including magnetic coils 256, polepieces 258, 260, and an armature 262. A flapper arm 264 is secured tothe armature 262 and extends perpendicular therefrom. The magneticassembly 254 is secured to a housing 270 of the valve 106.

The flapper 264 has a mounting location 274 about which a flexure tube276 pivotally secures the flapper 264 to the housing 270 through a pressfit. The flexure tube functions to position the armature 262 between thepole pieces 258, 260 and to provide a fluid seal between the magneticassembly 254 and the flapper valve 250 and spool valve 106.

The valve 106 includes a spool 280 slidably mounted in a chamber 282.Each end of the spool 280 has an associated pilot chamber 284, 286,respectively. The pilot chamber 284 is in fluid communication with thesupply pressure P_(s) through a fixed orifice 290. The pilot chamber 286is in fluid communication with the supply pressure P_(s) through a fixedorifice 292.

The pilot chamber 284 is in fluid communication with an associated pilotorifice 296. The pilot chamber 286 is in fluid communication with anassociated pilot orifice 298. The flapper 264 is position at a locationequal spaced from the pilot orifices 296, 298 when the armature 262 iscentrally located between the poles 258, 260. In such a condition, fluidflows through both pilot orifices 296 and 298 equally so as to providean equal pressure on both sides of the spool 280. The spool 280 undersuch a condition assumes a centered position as shown in FIG. 4. Flow offluid through the orifices 296, 298 is referred to as first stage flow.

The spool 280 includes spaced apart land sections 302, 304, spaced by aportion 306. When the spool is in its centered position as shown in FIG.4, (i) the land 302 blocks fluid communication of chamber 282 from apassage 310 and with a passage 311 connected to the reservoir 120, (ii)land 304 blocks fluid communication of chamber 282 from a passage 312,and (iii) a passage 314 is in communication with the chamber 282 betweenthe lands 302, 304. A feed back wire 320 operatively connects the end ofthe flapper 264 with the land 302.

The passage 314 is in fluid communication with the fluid chamber 28 ofthe actuator 12. The passage 310 is in fluid communication with theaccumulator 100. The spool 280 when in the centered position as shown inFIG. 4, blocks fluid communication between the pump 104 and the fluidchamber 28 of the actuator 12.

When in the flapper 264 is in the centered position, pilot fluid fromthe pump 104 communicates through orifices 290, 292, through nozzles296, 298, through passage 310 to the accumulator 100. This pilot flow isreferred to as a quiescent fluid flow of the first stage valve.

When the controller 140 applies an electric current to the coils 256 inone direction, the armature 262 pivots about the mounting location 274due to the magnetic attraction of the pole pieces 258, 260 on thearmature 262. This pivoting of the armature 262 causes the flapper 264to move toward one of the pilot nozzles 296, 298 depending of thedirection of pivot. As the flapper 264 moves closer toward the nozzle296, the fluid pressure in chamber 284 increases and the fluid pressurein chamber 286 decreases. This pressure imbalance causes the spool 280to move to the right, as viewed in FIG. 4, thereby permitting fluidcommunication between the pump 104 and the chamber 28 of the actuator12. The feedback wire 320 tends to move the flapper 264 away from thenozzle it is closest to in response to movement of the spool 280. Theamount of fluid communication is a function of the amount of current tothe coils 256.

Control of the position of first member 20 of the actuator 12 throughvalve assembly 105 provides control for response to low frequency orinertia inputs to the system. For example, when the vehicle rounds acorner, inertia forces transfer loading from the inside of the vehicleto the outside of the vehicle. As weight is transferred, strutdisplacements change according to the apparent stiffness of theaccumulator 100.

Control of the electrically controlled valve assembly 101 for highfrequency inputs is depended upon the operating speed of the valveitself. If the valve assembly 101 has a "slow" operating time, i.e.,greater that fifty milliseconds, the valve opening is controlled inresponse to an estimate of the roughness of the road surface and anestimate of the handling inputs. Handling inputs can be characterizedeither from the accelerations sensed by the CG sensors 142 or directlyfrom steering input sensors while road roughness is derived fromvariation in the pressure P_(i) in the actuators 12.

If the operating speed of the valve assembly 101 is less than 10milliseconds, control of the valve is accomplished in response to sensedspecific road inputs.

Referring to FIG. 5, a control process 400 followed by the controller140, in accordance with a preferred embodiment of the present invention,is depicted. In step 402, the controller is initialized upon power up ofthe vehicle. In step 404, the pressure P_(s) of the pump is monitored.In step 406, the controller makes a determination as to whether P_(s) isequal to a predetermined value X. If the determination is negative, theprocess proceeds to step 408 where the pressure output of the pump isregulated so as to make the P_(s) equal to X.

The determination in step 406 and the adjustment of the pump outputpressure in step 408 assumes that the output of the pump 104 isconnected to an electrically controlled pressure regulator or that thepump output pressure is electrically controllable in one of severalknown ways. It is also possible to have a load sense variabledisplacement pump arrangement. In such an arrangement, the pump may be aswash plate pump with a load sense feedback line. The angle of the swashplate and, in turn, the pump output pressure is controlled in responseto the sensed output pressure of the pump through the load sensefeedback line. If a swash plate pump with load sense feedback is used,steps 406, 408 can be eliminated from the process followed by thecontroller 140.

The process then proceeds from step 408 to step 410. If thedetermination in step 406 is affirmative, the process proceeds to step410. In step 410, the controller monitors the CG sensors and thepressures sensed by sensors 134, 136. In step 412, the desired strutvelocity is determined in accordance with a desired predeterminedhandling characteristic of the vehicle and a predetermined ride feel forthe vehicle.

In step 414, the controller separates the velocity demands into bothhigh and low frequency demands. Specifically, the controllermathematically hi-pass filters the monitored pressure signal P_(i). Itwill be appreciated that an analog hi-pass filter can be used betweenthe sensor 136 and the controller 140. If there is a high frequency roadinput force present, a determination is made in step 416 as to whetherthe pressure signal indicates that the motion of the actuator 12 is loadaiding.

A motion is load aiding if:

    (P.sub.A -P.sub.i)×V.sub.demand >0

where V_(demand) is positive for downward movement and negative forupward movement of the second member 20 as viewed in the FIGS. If themotion is load aiding, valve 101 is controlled in step 418 in accordancewith a predetermined ride feel and handling characteristic.

In step 414, the controller also mathematically low-pass filters themonitored pressure signal P_(i). It will be appreciated that an analoglow-pass filter can be used between the sensor 136 and the controller140. Such low frequency adjustment includes roll, pitch, passengerloads, luggage, etc.

If the controller determines that the road input force is low frequencyin step 414 or if the determination in step 416 is negative, a controlsignal is developed by the controller 140 by adding a control signalvalue functionally related to a desired velocity demand to a bias valueat step 420. The bias value is a value determined by the controller 140which is necessary to maintain an average ride height so that, incombination with a predetermined actuator size, the mean value pressurein the accumulator 100 is at approximately one-half of the systempressure P_(s). Both of the first stage flows through the pilot orifices196, 198, 296, and 298 flow to the accumulator 100. There is acontinuous leakage through the open center valve 102, through thepassage 212, into the chamber 28, through the manifold 42 and to thereservoir 120. The orifice are designed so as to maintain the pressureP_(A) in the accumulator 100 at approximately one-half the systempressure P_(S). If the leakage path is insufficient for this purpose,i.e., the pressure P_(A) is greater than one-half P_(s), the bias signalprovided by the controller 140 positions the valve 106 more to the left,as viewed in FIG. 4, so as to provide a larger leakage path toreservoir. The control signal from step 420 controls the valve 105 instep 422.

In accordance with another embodiment of the present invention, the roadroughness is determined through Fourier analysis or an "averaged"high-pass filter of the signal P_(i) from each of the pressure sensorsfor the actuators 12 for each of the corners of the vehicle. Signalsfrom the CG sensors are monitored for handling characteristics such assignals indicative of the lateral acceleration of the vehicle andlongitudinal acceleration of the vehicle. The control signals to theassociated valves 101, 105 are provided in response to the roadroughness determination and desired handling characteristic.

From the above description of a preferred embodiment of the invention,those skilled in the art will perceive improvements, changes andmodifications. Such improvements, changes and modifications within theskill of the art are intended to be covered by the appended claims.

Having described a preferred embodiment of the invention, the followingis claimed:
 1. An actuator for a vehicle suspension system comprising:afirst member connectable to one of a sprung mass and an unsprung mass ofthe vehicle, said first member having an opening with an inside wall ofpredetermined dimension; a second member connectable to the other thesprung mass and the unsprung mass of the vehicle, said second memberhaving an outside wall of a predetermined dimension; said dimension ofsaid outside wall of said second member being adapted so as to betelescopically received within said opening of said first member therebydefining a fluid chamber; means for connecting a source of pressurizedfluid to said fluid chamber; fluid seal means located near an end ofsaid first member for forming a fluid seal between said first member andsaid second member; bearing means operatively mounted between saidinside wall of said first member and said outside wall of said secondmember for maintaining said inside wall of said first member spaced formsaid outside wall of said second member thereby defining a fluidmanifold between said inside wall of said first member, said outsidewall of said second member, said bearing means, and said fluid sealmeans, said bearing means permitting fluid pressure in said fluidchamber to communicate with said fluid manifold; means for connectingsaid fluid manifold to reservoir, so as to maintain continuous fluidcommunication between said fluid manifold and said reservoir, and meansfor providing a continuous flow of fluid into said fluid chamber, aroundsaid bearing means, into said fluid manifold, and to said reservoir andmaintaining fluid pressure in said fluid chamber at a predeterminedvalue.
 2. The actuator of claim 1 wherein said fluid seal means is a lowfriction, low pressure seal.
 3. The actuator of claim 1 wherein saidbearing means includes first and second spaced apart bearings.
 4. Theactuator of claim 1 wherein said first member is connected to saidsprung mass and said second member is connected to said unsprung mass.5. The actuator of claim 1 wherein said inside wall of said first memberand said outside wall of said second member are both cylindrical inshape.
 6. The actuator of claim 1 wherein said means for providing acontinuous fluid flow is a two stage servo valve with first stage fluidflow connected to said fluid chamber.
 7. A vehicle suspension systemcomprising:an actuator connectable between chassis and a wheel, saidactuator having a variable volume fluid chamber, displacement betweenthe chassis and wheel being controlled by pressure in said chamber; apump; an accumulator; means to pressurize said accumulator from saidpump to a predetermined pressure; sensing means for sensing fluidpressure in said actuator chamber and in said accumulator, fluidpressure in said chamber being indicative of force inputs to thevehicle; an electrically controlled valve for connecting a selected oneof said pump and said accumulator to said chamber; and control means forcontrolling said electrically controlled valve in response to forceinputs and a pressure differential between said accumulator and saidchamber.
 8. The suspension system of claim 7 wherein said control meansincludes means for determining a velocity demand motion value for saidactuator in response to a sensed force input, and means for determiningif the force input on the actuator would result in a load aiding motionof the actuator or load opposing motion of the actuator, said controlmeans connecting said accumulator to said chamber of said actuator ifsaid resulting motion is determined to be load aiding and connectingsaid pump to said chamber of said actuator if said resulting motion isdetermined to be load opposing.
 9. The suspension system of claim 8wherein said control means determines a resulting motion is load aidingif:

    (P.sub.A -P.sub.i)×V.sub.demand >0

where P_(A) is the fluid pressure in said accumulator, P_(i) is thefluid pressure in said actuator and V_(demand) is positive for relativemovement of the unsprung mass and the sprung mass apart from each otherand negative for relative movement of the unsprung mass and the sprungmass toward each other.
 10. The suspension system of claim 7 whereinsaid actuator is a reduced friction actuator having continuous leakageof fluid from its fluid chamber to a reservoir.
 11. The suspensionsystem of claim 7 further including means for maintaining fluid pressurein said accumulator to one-half the pressure output from said pump.